1. Field of the Invention
The invention relates to a device for detecting a pulsating variable and for generating a variable-dependent signal corrected for pulsation error and relates in particular to a device for detecting the pulsating mass, or flow rate, of the air aspirated by an internal combustion engine and for obtaining the mean air mass taken in per segment corrected for pulsation error.
2. Description of the Related Art
For detecting the flow rate of air aspirated by an internal combustion engine, micromechanical air flow rate meters are typically used, whose sensor element is exposed to the flowing air. Since under certain conditions the air mass, or air flow, in the intake tube of the engine is in the form of a pulsating flow in the intake tube, the sensor element is installed for instance not directly in the intake tube but rather in a flow bypass, in which the pulsations are less pronounced. In some engines, such as throttle-free engines, however, the intake tube pulsations are especially strong and cannot be compensated for unless further provisions are made.
There are physical limits to the aerodynamic compensation for pulsation errors. In defining the bypass geometry of the air flow rate meter, the compensation is furthermore limited by the thermal inertia of the sensor. The pulsation error depends on the parameters of the mean air flow rate and the frequency and amplitude of the pulsations. For ranges of parameters in which the bypass geometry cannot compensate for the thermal error of the sensor, in the final analysis incorrect indications remain because of intake tube pulsations. To reduce these incorrect indications, it is proposed for instance in Published, Nonexamined German Patent Application DE-OS 44 08 243 that along with a pneumatic pulsation damping, additional electronic filters are used, which suitably vary or damp the output signal of the sensor element. The electronic filter is a constituent of the sensor and precedes a circuit for characteristic curve adaptation and characteristic curve limitation; this circuit for characteristic curve adaptation and characteristic curve limitation is also a constituent of the sensor element. The electronic filter must be adapted to the expected signal in such a way that both high-frequency natural oscillation and the fundamental oscillation of the pulsation are damped, so that at the output of the filter, the remaining pulsation amplitude is only slight. From the signal thus obtained, mean values of the pulsating variable, that is, mean values of the aspirated air mass, are calculated in a following microcomputer, such as the control unit of an internal combustion engine.
It is an object of the present invention to provide an improved device for detecting a pulsating variable and for correcting for pulsation of the pulsating variable.
It is a further object of the present invention to provide a device for generating a digital signal characteristic of an amount of air taken into an internal combustion engine from a pulsating airflow during a segment or pulsation period of the engine, which is effectively corrected for pulsation error.
It is another object of the present invention to provide a sensor unit for generating a digital signal characteristic of an amount of air taken into an internal combustion engine that is corrected for pulsation error.
According to the invention the device for generating a corrected signal dependent on an amount of air taken into an internal combustion engine during engine operation, which is corrected for pulsation error due to pulsation of the air taken into the internal combustion engine, includes
a sensor element with an analog signal processing means for generating an uncorrected analog signal in cooperation with the sensor element, which varies according to an air flow rate of said air taken into the internal combustion engine;
digitizing means for digitizing the uncorrected analog signal to form an uncorrected digital signal;
analyzer means for determining an amplitude of the uncorrected digital signal;
means for determining a mean value of the uncorrected digital signal;
memory means with at least one pulsation error characteristic curve stored therein, which represents a functional relationship between the pulsation error and sensor signal parameters, which include average air flow rate, pulsation frequency and pulsation amplitude;
means for determining the pulsation error from the at least one pulsation error characteristic curve and at least one of the sensor signal parameters; and
means for correcting the mean value of the uncorrected digital signal with the resulting pulsation error.
In a preferred embodiment of the invention the device is a sensor unit including the sensor element so that the mean value is delivered to the control thus relieving the control unit of the necessity of digitizing and correcting the analog signal from the sensor element. Alternatively, the digitizing and analysis of the analog signal from the sensor element can take place in the main control unit of the internal combustion engine.
The device according to the invention for detecting a pulsating variable has the advantage that especially reliable compensation for the pulsation error is obtained, and this can advantageous be adapted to different engines and functions reliably under all possible conditions. In an especially advantageous embodiment the pulsation compensation is already performed in the sensor unit, which relieves a following evaluation device, such as the microprocessor of an engine control unit.
Advantageously, a pulsation error characteristic curve or graph that is taken into account in the compensation is stored in the sensor unit itself. This pulsation error characteristic curve can be ascertained, for instance, at a suitable test bench and can be written into a memory of the sensor unit before the engine is put into operation. The pulsation error characteristic curve PK is a function of various variables; for instance, PK=f (airflow rate, frequency, amplitude). The principle is equally applicable if a corresponding pulsation error performance graph without correction in the HFM is stored in the control unit.
The further advantages of the invention obtained by the provisions recited in the dependent claims. Independently of these advantageous features, there can also be a pulsation error characteristic curve in the following evaluation device, or in other words in the control unit, and further or additional corrective provisions can be performed in the control unit. The invention is advantageously employed for detecting the air flowing from an engine or internal combustion engine through the intake tube, in which a hot-film air flow rate meter is disposed as a sensor in the intake tube.